Various plastic powder coating techniques are known for developing fusion-bonded plastic finishes. One common technique is electrostatic spraying in which plastic powder particles are electrostatically charged and sprayed toward a metal article that has been grounded, causing the particles to cling to the article until fused.
It is important that the size of the powder particles be carefully controlled since those that are too small (so-called "fines") tend not to cling to the article and constitute dust that must be dealt with. Typically, particles having a maximum dimension on the order of about 10 microns or less are considered fines, although the minimum size may depend to some degree on the particular compound involved, as those skilled in the plastic coating field will appreciate. Knowing this, manufacturers of plastic powder coating materials strive to select and control their manufacturing techniques in such way as to minimize the generation of fines and thus maximize their yield of usable product.
Traditional approaches to producing powder coating materials have depended to a large degree on whether the material being processed is a thermoplastic compound or a thermosetting compound, but involve generally the formation of melt-blended particles in either pellet or flake form that are fed to a grinder and pulverized to powder. The pellet form of feed material for the grinder is well recognized as being preferred over the flake form for a number of reasons. First, pellets are fairly free flowing in bulk and thus feed well into the grinder without binding. Flakes are far less flowable in bulk and tend to bridge in the feed system and, in extreme cases, jam the throat of the feed system requiring costly and labor intensive dismantling of the feed system to clear the jammed material.
Another recognized advantage pellets offer is that they can be made to have a generally uniform size and as a result are ground uniformly by the grinder with minimal fines generated. Flakes, on the other hand, are generally non uniform in size and much larger than pellets. The non-uniformity requires that the larger flakes be recirculated through the grinding chamber more often than the smaller flakes. The more often a flake is recirculated through the grinder, the more likely fines are to be generated. Consequently, the flake form tends to generate a higher percentage of fines when ground then does the pelletized material.
For reasons to be explained below, the processing technology available prior to this invention, insofar as applicant knows, has permitted only thermoplastic compounds to be produced in the desirable pelletized form and has required that thermosetting materials produced at a mass production rate be in the less desirable flake form.
One of the key characteristic differences between thermoplastic and thermosetting compounds that has contributed to the disparity between the processing technology available for these materials is that thermosetting compounds inherently take on a permanent set when heated above their cure temperatures and as a result must be carefully managed when processed in the molten state. Once cured, the material cannot be remelted and, if the material sets within processing equipment, can cause major problems which are expensive to overcome. Thermoplastics, on the other hand, can be reheated and reformed a number of times without significant change and, if hardened in a processing machine, can simply be reheated to overcome the problem.
Another major characteristic difference between thermoplastic and thermosetting compounds is that the latter tend to be considerably more brittle and prone to fracture during processing, which can lead to a higher production of undesirable fines in the processing operation.
The traditional approach to manufacturing pelletized thermoplastic powder coating compounds involves first pre-mixing the individual dry ingredients in a mixer, such as a typical high intensity mixer having either a fixed or removable bowl, to homogenize the ingredients. The dry mixture is then fed to a compounding extruder which melts the material, thoroughly blends it, and discharges it in molten form. Since thermoplastic compounds do not take on a permanent set when heated to high temperatures, it is common to process the material through the extruder at temperatures well above the melting temperature of the compound (e.g., typically 350 to 450.degree. F. for some common thermoplastics), but below the charring temperature, in order to take advantage of a corresponding increase in the flowability of the material at the higher temperature. U.S. Pat. Nos. 3,195,868; 3,423,074; 3,564,650; and 3,642,406 disclose various compounding extruders of the general type that may be used to process thermoplastic compounds. The above patents and their disclosures are incorporated herein by reference.
The molten thermoplastic in the extruder is typically forced through a die to form strands of extrudate which are then pelletized in one of several ways including strand, hot face, and underwater pelletizing, a brief description of each being given below.
Strand pelletizing involves drawing the soft strands through a water quenching bath and then severing them into pellets as they exit the bath. The water is blown off the strands at the cutter. This process requires that the hot strands exhibit a certain amount of melt strength that allows them to be pulled through the water bath. Thermoplastic compounds typically possess the requisite melt strength, whereas thermosetting compounds, to my knowledge, do not. Consequently, I do not consider this pelletizing process to be available for thermosetting polymers and compounds.
The hot face pelletizing process utilizes a die having radially extending extrusion ports through which the material is extruded in strand form. A cutter rides against the radial face of the die and severs the hot extrudate strands into pellets. As the pellets are cut, they are thrown into an adjacent chamber where they are cooled and transported away by either water or air. A typical hot face pelletizing process is disclosed in the aforementioned U.S. Pat. No. 3,642,406. This process requires that the material exhibit a certain amount of shear strength in the extruded state which enables the strands to be severed cleanly by the cutter. Thermoplastic compounds possess this property, whereas thermosetting compounds do not. Thermosetting compounds typically have a characteristically low viscosity in the molten state which causes the material to smear across the die face when contacted by the cutter, rather than being cleanly severed like the thermoplastic material.
Underwater pelletizing is similar to hot face pelletizing except that the die and cutter are immersed in a water bath. As the strands exit the die, they are quenched and cut almost simultaneously into pellets, and as they exit and are cooled by the water they develop a generally spherical shape. It is to be noted that the generally spherical shape of the pellets is considered the most desirable of the pellet forms, since the additional roundness of the particles increases their bulk flowability. The pellets are carried away by the water to a pellet separator and then dried in a conventional centrifugal dryer. A typical design of dryer for thermoplastic polymers incorporates a vertically disposed high speed rotary blade mounted within a housing and an air stream passing upwardly through the housing. The pellets and any cooling water clinging to them are fed into the housing adjacent to its lower end and are hurled by the high speed blade against the wall of the housing and from there fall to the bottom of the dryer where they are collected. The water entrains in and is drawn upwardly with the air stream through a top discharge.
Although the described thermoplastic polymer pelletizing process works well for processing thermoplastic compounds, it is not used, to my knowledge, for processing thermosetting compounds. The inherent brittleness of thermosetting compounds causes them to fracture and be generally pulverized to fines upon impact with the rotary blade and housing of such a centrifugal dryer, making the material unsuitable as feed stock for the grinder.
Because of the above limitations in current processing technology, the traditional approach for processing thermosetting compounds has been in flake form and involves pre-mixing the individual ingredients and feeding them to an open-ended compounding extruder where the material is melted, thoroughly blended, and discharged in somewhat molten form to either a chill roll or cooling belt device which cools and flakes the material. The chill roll device employs two counter-rotating chill rolls between which the extrudate issues to squeeze and cool the material into a thin sheet form. Once cooled, the sheet is fed past a roller outfitted with pins which fracture the sheet into flakes or chips. The cooling belt apparatus similarly has two chill rolls which squeezes and cools the extrudate into a thin sheet form. The sheet exiting the chill rolls is deposited onto a cooling conveyor belt which is of considerable length and, when the strip is sufficiently cooled, it is broken into flakes in similar fashion.